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is a difficulty which Professor Ray Lankester expressed so long ago as 1876, when contrasting Mr. Spencer's theory with that of Pangenesis.* And now we are certainly entitled to go further, and to enquire why the effects of constantly performed mutilations, which are not repaired, do not tend to become inherited after many generations, as they certainly ought to be on Mr. Spencer's theory. Finally we ask-and ask in vain-for an explanation of the fundamental facts of atavism. +
Mr. Darwin's “Provisional Hypothesis of Pangenesis” differs very fundamentally from the theory we have been hitherto considering. It distinctly recognises the necessity of grappling with the two questions which we have seen to be the fundamental problems of heredity—the problems of transmission and of development-and it answers these questions in a clear and unmistakable manner.
Mr. Darwin supposes certain organic particles called gemmules to be the specific bearers of hereditary tendencies. These are continually—at all stages of development-being given off by every cell in the body. The gemmules are continually circulating throughout the body, and finally collect in the reproductive cells. Each gemmule is a representative of the cell from which it took its origin (at that period of its life-history), and is capable, under proper conditions, of developing into a similar cell. The germ-cells, then, being simply collections of such gemmules, possess the potentialities of new organisms exactly similar to the old. This is Mr. Darwin's solution of the problem of transmission. The problem of development he solves as follows. The gemmules representing any given stage of development have a special affinity for the partially-developed cells of the stage immediately preceding. They seek out and unite with these, and are thus able to develop in the right order their corresponding parts. This extremely ingenious suggestion Mr. Darwin supports by reference to the unerring accuracy with which pollen of the right species alone develops pollentubes when a number of different kinds are placed on the stigma of a flower.
The phenomena of atavism, Mr. Darwin explains by supposing that some gemmules lie dormant for
more or sewer generations, and suddenly, owing to especially favourable conditions, are enabled to develop into their corresponding structures.
It will be seen that this theory explains the problem of transmission mainly by the hypothesis of the redevelopment of new gemmules from the cells of the body into which the former gemmules were developed. It explains the problem of development, by supposing that there has been a preformation * of distinct germs of structure, and that by means of the presence of these the germ-cell is able to produce the organism.
Mr. Darwin's theory explains with great readiness most of the observed abnormal phenomena of heredity, but it was to explain the supposed hereditary effects of use and disuse that it was especially constructed. This was indicated in the last paper (p. 210), and it was there pointed out that Mr. Darwin was right in his belief that a theory on the lines of Pangenesis would have to be ultimately adopted, if acquired characters are inherited. Its explanation of the problem of transmission by the assumption of the redevelopment of the gemmules from the cells of the body alone enables it to explain the hereditary transmission of the modifications acquired by those cells. The other theories of heredity which profess to allow of such transmission—such as Mr. Spencer's, depending on assumed absolute solidarity of the organism, or the late Professor Nägeli's, depending on cyclical development of idio. plasm, which we shall consider presently—are open to even more serious and fundamental objections than is Pangenesis. We are, I think, in a position to say that if acquired characters are transmitted, a theory of heredity not essentially differing from Pangenesis will have to be adopted, and the difficulties—very great difficulties-attaching to it will have to be explained away somehow.
The theory of Pangenesis has been often and severely criticised; it is not my purpose to give a list of these criticisms here, but I shall consider three which seem to me to be of special importance. First, there is the argument about the inconceivable number of gemmules that must exist in the fertilised egg-cell
* “Advancement of Science,” pp. 280-282. + It is hardly fair to Mr. Spencer to charge him, as is so often done, with being too vague and general in his theory of heredity. In the first place, he is not in the least vague, and the impression that he is usually arises from failure to thoroughly understand his meaning. That his theory is
general," is true, as we have already insisted: but it should be remembered that he expressly says " a positive explanation is not to be expected in the present state of biology. We can look for nothing beyond a simplification of the problem.” It is, however, quite open to us to criticise the theory on the ground of its being inconsistent with known facts; and further, to demand now a fuller and more special explanation of the phenomena of heredity, as the result of the many years of research which have elapsed since Mr. Spencer's words were written.
| "An organic being is a microcosm-a little universe, formed of a host of self-propagating organisms, inconceivably minute and numerous as the stars in heaven (“ Variation under Domestication," vol. ii. p. 399).
* Harvey, following Aristotle, enunciated the theory of epigenesis, to explain the process of development of the fertilised ovum into the adult organism. He believed that the various structures of the adult were formed by the successive differentiations of a relatively homogeneous rudiment (the fertilised ovum). Malpighi contradicted this on the ground of having directly observed the body of the chick in the egg during the early days of incubation. He unjustifiably assumed that it existed in the egg as a whole before incubation. Later on his views were taken up and extended by Bonnet and Haller, and became widely accepted. Bonnet, however, modified his doctrine in later life, and looked upon the egg as being only an "original preformation” of the body, not necessarily an actual miniature of the latterC. F. Wolff, in 1759, entirely exploded the theory of "evolution,", or "preforma
as generally understood, and re-established "epigenesis;" but Darwin in his theory of pangenesis undoubtedly revived the conception of the preformation of distinct germs of structure, though not, of course, the idea of a miniature organism in the ovum, but rather a conception akin to Bonnet's later views. It is in this modified sense that we use the word "preformation” and contrast it with pure "epigenesis,” to which Professor Weismann has conspicuously returned.
of, for instance, a mammal. This is an old objection imagine that each single stage of a part is present in to Pangenesis, and one to which Mr. Darwin replied the germ, as a distinct group of gemmules, seems to by pointing to the enormous number of molecules me to be a childish idea, comparable to the belief that which probably exist in a cubic millimetre of water. the skull of the young St. Lawrence exists at Madrid, But when we consider, first that a gemmule is very while the adult skull is to be found in Rome. We much larger than even a very large molecule of a are necessarily driven to such a conception if we complex organic substance, and secondly try to get assume that the transmission of acquired characters some conception of the enormous number of cells takes place."* which will have to be represented, the objection These three lines of argument show the heavy appears of considerable force.
batteries which can be brought to bear against The second argument I propose to consider is that Pangenesis from various sides. We can hardly furnished by Mr. Francis Galton's and Professor accept such a theory unless no other is possible. Romanes' experiments on the transfusion of blood and Various attempts at modifying it have been made, transplantation of tissues in rabbits and guinea-pigs. of which, perhaps, the most important is Mr. W. K. Mr. Galton performed extensive experiments in trans. Brooks'.t I shall not, however, discuss it here, but fusing the blood of distinct varieties of rabbits. He refer the reader who is interested in the matter to obtained no hereditary effect. Mr. Darwin would not Professor Weismann's very complete criticism of it admit that these experiments negatived the doctrine (" Essays on Heredity" pp. 326–332). of Pangenesis. It was no essential part of his theory,
(To be continued.) he argued, that the gemmules should use the circulation as a means of transit. Obviously they could not do so in many of the lower animals and in plants.
ROSSENDALE RHIZOPODS. But in reply we may'argue, with Professor Weismann, that it is difficult to see why the gemmules should fail
No. 6. to take advantage of so favourable a means of transit, and also how they could contrive to avoid it.
LL the Rhizopods figured and described in my A
previous articles, have belonged to the subProfessor Romanes has since repeated these experi
order Lobosa-having lobose, or finger-like pseudoments with great care, and has also carried out very
podia, and containing many forms familiar to most complete skin-transplantation experiments on guinea-microscopists
. I have now to describe some less pigs of distinct varieties, again with negative results.
familiar Rhizopods, belonging to the sub-order Now in this case, on the hypothesis of Pangenesis, a
Filosa. In this sub-order, the sarcode is not so certain number of gemmules must have been present in
obviously separable into a clear ectosarc and a more the transplanted strips of skin, and how they failed
granular endosarc, but apparently consists wholly of to produce any effect is not at all clear.
the latter ; and probably on this account the pseudoPerhaps the most trenchant criticism of Pangenesis
podia, instead of being lobate or finger-like, are in is Professor Weismann's, and it is best given in his
the form of exceedingly delicate, forked threads, own words. It is a criticism of Darwin's solution of
which become finer and finer as they branch, and the problem of development by the hypothesis of
are usually more numerous than those of the lobose preformation. “One and the same part of the body
Rhizopods. The sub-order contains seven genera, must be represented in the germ or sperm-cell by
five of which are represented in this district, and many groups of gemmules, each group corresponding
about seventeen species, some of which are beautiful to a different stage of development; for if each part
objects, the tests being made up of round, or oval, gives off gemmules, which ultimately produce that
over-lapping plates, arranged in such a way as to part in the offspring, it is clear that special gemmules
form definite patterns. must be given off for each stage in the development of
Pamphagus mutabilis, perhaps the most interesting the part, in order to reproduce that identical stage.
species of the genus, I have not yet met with ; but But the ontogeny of each part is in reality continuous,
another species, P. hyalinus, is not uncommon in our and is not composed of distinct and separate stages.
clear ponds and wells, though from its minuteness We imagine these stages as existing in the continuous
and inconspicuous character, it may readily be overcourse of ontogeny ; for here, as in all departments of
looked. On a side view it is sub-spherical, and the nature, we make artificial divisions in order to render
lower end is produced into a short neck, with a cir. possible a general conception, and to gain fixed
cular mouth, through which the long, fine pseudopoints in the continuous changes of form which have,
podia are protruded. As generally seen, it appears in reality, occurred. Just as we distinguish a sequence
as a roundish granular mass, greyish in colour, of species in the course of phylogeny although only a
occasionally with cloudy yellow patches in the gradual transition, not traversed by sharp lines of
interior, and in specimens from some localities demarcation, has taken place, so also we speak of the stages of ontogeny, although we can never point out
• “Essays on Heredity," pp. 316-317. where any stage ends and another begins. To
† “Law of Heredity,” 1883.
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